Search found 17 matches

by Papalapa
Mon May 13, 2019 9:14 am
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

I’ve got an issue 3 board (possibly produced for the French market given that it has a transistor inverting the video signal to the modulator) that has a resistor as mentioned in the posts above. Mark Just a silly question: on your pictures I see that the resonator is soldered in the opposite direc...
by Papalapa
Mon May 13, 2019 9:03 am
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

Maybe it’s just a broken ULA like 99% of all the repairs in this forum. It seems to be the weak spot. Yes it is possible but I want to explore first the "easy solutions" because the standard ULA is not easy to find and it seems that Andy has not more units ready at the moment. On the other hand min...
by Papalapa
Sun May 12, 2019 8:39 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE. That would indicate a problem with the ceramic resonator, surrounding passive components, or ULA. But have a look at the board: there's some bodge resistor the...
by Papalapa
Sun May 12, 2019 8:29 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

Test the Z80 /MREQ, /RFSH, /RESET, A14, A15 and A6 pins with your logic probe. There is a pin out on the forum, check the link in my sig below. Mark Thank you Mark, I was aware about your post with the pin-out of the chips, in fact I made a PDF document with all the information in just one page and...
by Papalapa
Sat May 11, 2019 1:42 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE. I don't know if these cheap test probes are reliables but I don't have oscilloscope. At the moment I ordered the 6.5MHz crystal because it's very cheap and easy...
by Papalapa
Sat May 11, 2019 11:29 am
Forum: Hardware
Topic: Dead ZX81.
Replies: 25
Views: 3616

Re: Dead ZX81.

flaviosr wrote:
Fri May 10, 2019 9:04 pm
Heppy to help!
Tell me the results! :D
Unfortunatelly after replacing both transistors the problem remains. Now I'll start to check the ULA and CPU chips and see what happens..
by Papalapa
Wed May 08, 2019 9:18 pm
Forum: Hardware
Topic: White screen and no cursor at all
Replies: 17
Views: 434

Re: White screen and no cursor at all

Well, I already replaced TR1 and TR2 unsuccesfully. They were replaced by exactly the same transistor model but unfortunatelly the problem remains: white screen and no "K". This week-end I'll start to investigate with the logic probe if the ULA or Z80 are damaged. Bad luck :(
by Papalapa
Tue May 07, 2019 2:53 pm
Forum: Hardware
Topic: Dead ZX81.
Replies: 25
Views: 3616

Re: Dead ZX81.

Obviously the RAM is ok. Having no more ideas I have decided simply to change the two transistors and the two capacitors and... the ZX81 is not perfectly working! I have chenged them all together so I will never know if it was a cold welding and which component was the bad one. The strange issue is...
by Papalapa
Mon May 06, 2019 3:18 pm
Forum: Hardware
Topic: Improving Timex/Sinclair Tape Loading
Replies: 145
Views: 21681

Re: Improving Timex/Sinclair Tape Loading

The components density on the board is quite low so, most probably a smallest PCB can be done and installed into the ZX81 case, and the voltage taken from the ZX81 itself that will avoid the batteries use.

When I get my ZX8 repaired I'll investigate about this...
by Papalapa
Mon May 06, 2019 3:00 pm
Forum: Hardware
Topic: Faulty ZX81 logic probe results
Replies: 11
Views: 1450

Re: Faulty ZX81 logic probe results

I'm in the same situation with my ZX81 at the moment: white screen with no cursor.

Which finally was the damaged part in your case, the ULA or the Z80 ? I think that do you replaced one of them, tested the ZX81 and replaced the other one later it isn't?